The present invention relates generally to the field of digital communication systems more particularly to phase recovery in a synchronous communication system.
The use of fiber optic communications continues to grow worldwide at a rapid pace. existing communications systems do not include digital and systems and methods for phase recovery to accommodate the expanding number of individual users.
Currently, fiber optic communication systems distribute signals, from a central office, through a fiber distribution system, to a number of different customers each at different distances from the central office. These systems use asymmetric bandwidths for both the downstream data (from the central office to the customer) and the upstream data (from the customer to the central office) directions. As an example, a communication system, such as Asynchronous Transfer Mode (ATM) system, uses a downstream link of 622.08 Mb/s and upstream data path of 155.52 Mb/s.
However, a problem arises in such ATM systems as the bursts of information travel along the upstream data link from the customers, at different distances, to the central office because the differences in distance create phase delays in the upstream data received at the central office. Additionally, this problem is usually compounded because many fiber optic communication systems use time division multiple access (TDMA) coding schemes. Under a TDMA coding scheme, the timing arrangement requires each customer to input a time variation in a time slot so that the information arrives at the central office in a timely fashion. This requirement is necessary because in a TDMA system if two or more customers from different directions send data through the upstream link, the customer that is farther away has to send his information sooner so that it falls in a time slot behind the customer that is closer. Again as in the non-TDMA case, each packet of incoming TDMA data at the central office is going to have a phase difference from the others. Thus recovering a clock reference from the upstream data is crucial for the proper synchronization and recovery of the upstream data because the recovered upstream data clock reference eliminates the effects of the phase delays on the upstream data.
A phase locked looped (PLL) has been used to recover this type of upstream data. An ATM type of system, as an example, requires that each packet of incoming data contains a preamble that allows a PLL to realign itself to each packet of incoming data. However, PLLs have numerous problems. The main problem is inadequate speed of signal acquisition. A PLL must adjust quickly to the incoming data packet when the size of the preamble is only a few bits long for an efficient transmission.
The customer premises equipment (CPE) units (telephones, PBX switches, etc.) receive the downstream clock, divide it by four to get the upstream clock, and then send the upstream data information back, in a synchronized fashion, to the central office equipment. The process of sending the upstream data information back to the central office introduces phase delays that effect the speed of signal acquisition of the PLL.
Thus, there is a need to develop an all digital phase recovery system (ADPRS) and method that uses the high speed downstream data clock to derive the upstream data clock in a fashion that is all-digital and would adapt very rapidly to the phase of each different packet of data as it comes in. However, at present, there is no such implementation.